This invention relates in general to power supplies and, in particular, to a plasma cutting torch power supply.
The metal working industry uses welding power supplies for joining metal members and structures. Plasma cutting is used to cut and prepare parts to be welded in some fashion in a welding process. Therefore, arc welding and plasma cutting are two processes which are frequently used together in many applications in the metal working industry. Both processes require power supplies which supply power of certain characteristics to welding torches or to cutting torches. To supply power for both processes, two separate power supplies have been used in conventional systems, one for performing plasma cutting and the other for performing arc welding. Plasma cutting requires a power source which provides power at high voltages and low current: for example, for cutting metal of up to one-half inch, the voltage and current supplied are typically at 100-120 volts and 20-50 amperes. On the other hand, arc welding requires a power source which can provide power at high current and low voltage, typically at 250 amperes and 20-30 volts for welding metal of up to one-half inch in thickness. Because of these different requirements for plasma cutting and arc welding, two separate power supplies have been used in conventional welding and cutting systems.
As explained above, welding requires a power supply which supplies high current at low voltage. The cutting process, however, requires a power supply for supplying low current but at high voltage. The output voltage of an ordinary welding power supply is not high enough for the plasma cutting process. Therefore, in conventional metal working, in order to perform plasma cutting, a completely separate plasma cutting power supply would have to be purchased. On construction sites where welding power supplies are engine-driven, there may not be any utility power at the proper voltage available for the plasma-cutting power supply. It is therefore desirable to provide a plasma cutting power supply where the above-described difficulties are not present.
In a conventional plasma cutting process, compressed air is normally used to both cool the torch and form a plasma through which an arc can pass to accomplish cutting. Compressed air is normally used since it is inexpensive and requires no special handling.
In the Heliarc 250HF Power Supply from L-TEC Welding and Cutting Systems of Florence, S.C., the voltage of the power supply required for the cutting process is lowered by adding argon to compressed air for cooling the torch as well as forming the plasma. By using a mixture of compressed argon and air, the resistance between the cutting torch and the workpiece is lowered, thereby also lowering the voltage required to sustain the cutting arc. Thus by changing the cutting process, L-TEC proposes a power supply where the voltage required for cutting is lower than that required for the conventional cutting process.
Using a mixture of compressed argon and air, however, can result in poor quality cuts. Also, an expensive gas such as argon is required. The power supply proposed by L-TEC is therefore not completely satisfactory. It is therefore desirable to provide an improved plasma cutting power supply in which the above-described difficulties are alleviated.